1. Field of the Invention
The present invention relates to an inner focus lens system and an image pickup apparatus using the same.
2. Description of the Related Art
Wide angle lenses of what is called a Gauss type configuration have been known as wide angle lenses for use in photographic cameras and still video cameras. The Gauss type lens system refers to a lens system having a substantially symmetric refractive power arrangement on both sides of an aperture stop. Japanese Patent Application Laid-Open No. 2003-241084 and Japanese Patent Application Laid-Open No. 2009-258157 describe Gauss type lens systems. The lens systems described in Japanese Patent Application Laid-Open No. 2003-241084 and Japanese Patent Application Laid-Open No. 2009-258157 are wide angle lenses having an F-number of approximately 2.8.
A lens system described in Japanese Patent Application Laid-Open No. 2009-237542 is a similar Gauss type lens system. The lens system described in Japanese Patent Application Laid-Open No. 2009-237542 is a standard lens having a large diameter (with an F-number of approximately 1.8). A lens system described in Japanese Patent Application Laid-Open No. 2010-66432 has a still larger diameter (with an F-number of approximately 1.4).
In the Gauss type lens systems described in the above-mentioned publications, a lens unit having a positive refractive power is arranged on the image side of the aperture stop, and this lens unit having a positive refractive power is advanced (namely, moved toward the object side) in its entirety. Thus, the Gauss type lens systems described in the above-mentioned publications are what is called rear focus lens systems.